1. Field of the Invention
The invention relates to luminance signal processing circuits, and more particularly, to a luminance signal processing circuit using a 3 line logical comb filter in magnetic recording reproducing devices such as video cassette recorders (hereinafter referred to as VCR).
2. Description of the Background Art
A VCR has been proposed using a comb filter, operating as a Y/C separating filter during recording for separating an input video signal into a luminance signal Y and a chrominance signal C (Y/C separation), and operating as a noise reduction circuit during reproduction for removing the small noise components in the reproduced luminance signal.
As an example of a comb filter, the so-called 3 line logical comb filter is known, such as the one described in U.S. Pat. No. 4,524,382, in which the comb type chrominance signal passing characteristic is implemented by using the bandpass filter (BPF) of each line for separating the chrominance signal components of 3 lines from the video signals of 3 lines to be logically processed.
In greater detail, a color TV signal (hereinafter referred to as video signal) of the NTSC system has the spectrum of luminance signal components and the spectrum of chrominance signal components alternately interleaved as shown schematically in the frequency spectrum diagram of FIG. 1 to avoid beat disturbance on the screen. Generally, when there is no difference between adjacent upper and lower horizontal lines, that is, when there is line correlation, luminance signal components are concentrated in the vicinity of the spectrum of n times (n is integer) the horizontal frequency f.sub.H (solid lines Y.sub.1), while chrominance signal components are concentrated in the vicinity of the spectrum of (n+1/2) f.sub.H around 3.58 MHz (broken lines C). That is to say, Y signal components and C signal components are mutually frequency interleaved by (1/2) f.sub.H. On the contrary, when there is difference between adjacent upper and lower horizontal lines, that is, when there is no line correlation, frequency spectrum regions around non-correlation components Y.sub.2 and Y.sub.3 of the Y signal are generated respectively in the low frequency region and the high frequency region of the whole band of signal Y in a frequency interleaved relation with the frequency multiplied output Y.sub.1 of the horizontal frequency (FIG. 1).
For example, in a VCR recording mode, a filter having a comb type band-pass characteristic periodically repeated in accordance with the repetition of the spectrum shown in FIG. 1 is required to extract chrominance signal components from the input video signal having such a frequency spectrum.
The above mentioned 3 line logical comb filter is widely used as the comb filter.
FIG. 2 is a block diagram showing an example of such a 3 line logical comb filter. Referring to FIG. 2, video signal (Y+C) applied to an input terminal T.sub.1 is provided to a BPF 20, as well as to a 1 H delay (1 HDL) circuit 15 to be delayed by 1 horizontal period. The output of 1 H delay circuit 15 is applied to a BPF 21 via an inverter 19, as well as to a delay circuit 17. The output from 1 H delay circuit 15 is also applied to a 1 H delay circuit 18, to be delayed by 1 horizontal period. The output of 1 H delay circuit 18 is applied to a BPF 22. Respective outputs of BPFs 20, 21, and 22 are applied to a vertical correlator 23, where the chrominance signal components C are extracted. Such a vertical correlator may be called as a vertical logical filter. Since the structure and operation of this vertical correlator 23 is disclosed in the aforementioned U.S. Pat. No. 4,524,382 and is well known, the description will be omitted here.
The chrominance signal C which is an output of vertical correlator 23 is applied to an output terminal T.sub.2 as well as to the negative input of a subtraction circuit 27 through a delay circuit 24. The delay circuit 24 fine-adjusts the delay time of the output of vertical correlator 23 for the purpose of making the positive and negative inputs of subtraction circuit 27 be in phase. To the positive input of subtraction circuit 27, the output of 1 H delay circuit 15 is applied via delay circuit 17.
The subtraction circuit 27 subtracts the chrominance signal applied to the negative input from the video signal applied to the positive input. That is to say, subtraction circuit 27 subtracts the phase-inverted chrominance signal C from the 1 H delayed video signal (Y+C) of 1 H delay circuit 15, whereupon a luminance signal Y is provided via terminal T.sub.5. The 3 line logical comb filter of FIG. 2 can be used as a noise reduction circuit for luminance signals during the reproduction by a VCR.
Using the aforementioned line correlation, a noise reduction circuit is proposed which suppresses the small noise components in the luminance signal generated during reproduction by the VCR. Such a noise reduction circuit functions so as to enhance the small signal components in advance that do not have line correlation during VCR recording, while removing the small signal that does not have line correlation as noise from the reproduced luminance signal during VCR reproduction.
FIG. 3 is a block diagram of such a noise reduction circuit. During VCR recording, a luminance signal Y separated by the above mentioned 3 line logical comb filter serving as a Y/C separating filter is provided to a terminal T.sub.6 to be applied to the positive input of a subtraction circuit 3 and a delay circuit 5, as well as to a 1 H delay circuit 2 via a switch 1 turned to the R side contact during recording. The 1 H delay circuit 2 provides the applied luminance signal Y to the negative input of subtraction circuit 3 after delaying the same by 1 horizontal period. The subtraction circuit 3 provides a difference signal by subtracting a delay signal of 1 H before from the current luminance signal, to a trap circuit 4. This difference signal corresponds to the difference components of the luminance signals between adjacent horizontal lines, that is, to the non-correlation components. The difference signal has the signal components of the chrominance signal band trapped at trap circuit 4, to be applied to a limiter 6, where the amplitude thereof is limited. As a result, the non-correlation small signal components are provided from limiter 6 to a K multiplication circuit 7 to be attenuated. The output of K multiplication circuit 7 is applied to one input of an addition circuit 10 via a switch 90 turned to the R side contact during recording. A luminance signal Y is applied to the other input of addition circuit 10 via the aforementioned delay circuit 5. This delay circuit 5 is provided for the purpose of making the timing of the two inputs of addition circuit 10 coincide with each other. The addition circuit 10 adds the non-correlation small signal components to the luminance signal Y. This output is provided through terminal T.sub.7.
Thus, switch 1 switches so as to select the luminance signal as the input of 1 H delay circuit 2 to carry out an open loop addition processing during VCR recording. As a result, the non-correlation small signal components in the luminance signal will be enhanced in advance.
During VCR reproduction, switch 1 is turned to the P side contact, whereupon the output of addition circuit 10 is fed back to 1 H delay circuit 2. Via subtraction circuit 3, trap circuit 4, limiter 6, and K multiplication circuit 7, non-correlation small signal components are inverted by an inverter 8, to be applied to one input of addition circuit 10 as the small noise components via a switch 9 turned to the P side contact during reproduction. As a result, addition circuit 10 removes the small noise components from the reproduced luminance signal to be provided from terminal T.sub.7. Thus, during VCR reproduction, switch 1 is switched so as to select the output of addition circuit 10 for carrying out the closed loop line correlation noise cancel operation.
In the 3 line logical comb filter of FIG. 2, two 1H delay circuits are necessary, while the luminance signal processing circuit of FIG. 3 functioning as a non-correlation component enhancement circuit during recording and a line correlation noise reduction circuit during reproduction requires one 1 H delay circuit. It is possible to simplify the structure of the circuit by sharing the 1 H delay circuits in the luminance signal processing circuit of FIG. 3 as one of the 1 H delay circuit of the 3 line logical comb filter of FIG. 2 and reduce the number of components in the circuit.
However, it is difficult to form the aforementioned non-correlation component enhancement circuit in the preceding stage of the 3 line logical comb filter because the 3 line logical comb filter functions as a Y/C separating circuit during VCR recording. Furthermore, if a noise reduction circuit is implemented in the succeeding stage of the 3 line logical comb filter during VCR reproduction, the aforementioned closed loop can not be established. Therefore, the sharing of the 1 H delay circuit has a disadvantage that there are many limitations associated with the structure of the circuit.